In Pak section is option No Underlined which treat files with '_' at the beggining as temporary.

Maybe good idea will be add an option to converter which can add this sign to file name or delete it. Sometimes I have lots of temporary files which could be needed in the future but I don't want it in version to tests. It takes huge amount of time to add '_' and then when I need files again delete '_' .

Ok, but what when I have images and I put them into _TEST subfolder and then I want create material from them? I'll drag and drop them on Material Editor window and then I'll have to cut and paste them in right directory where I want to create material.

No, the material file can store absolute paths to images, no need to have them in the same folder as the mtrl file.
Also, if you're creating test materials, why not put them into the _TEST subfolder? The default Data folder structure is just a recommendation, it's not hardcoded into the engine that materials must be in the Mtrl folder.

When you have gfx and mtrl files together it's always more clear to operate with them. At least for me And I don't mean test material but material which I wanna use in game. But for example after two months I want change normal maps in several materials. Now for me it will be time-consuming to move images form temp to material folder and than drag and drop them in Editor ;P

Edit: But when I looked closer to it, maybe it's a good solution xD I'll try it. When it won't be comfortable for me I'll write small program to convert this